effectful.ts

type Effect = { kind : string }
type EffectHandler<E extends Effect> = (e : E) => unknown

function runGeneratorSyncWithEffect<ResultT, YieldT extends Effect, ArgsT extends any[]> (
    effectHandler   : EffectHandler<YieldT>,
    func            : (...args : ArgsT) => Generator<YieldT, ResultT, any>,
    args            : ArgsT,
    scope?          : any
) : ResultT
{
    const gen       = func.apply(scope || null, args)

    let iteration   = gen.next()

    while (!iteration.done) {
        iteration   = gen.next(effectHandler(iteration.value))
    }

    return iteration.value
}


type EffectStateGet = { kind : 'state_get' }
type EffectStateSet = { kind : 'state_set', value : number }

const effectfulFunction = function * () : Generator<EffectStateGet | EffectStateSet, void, { value : number }> {
    const state = yield { kind : 'state_get' }

    yield { kind : 'state_set', value : ++state.value }
}

const state = { value : 0 }

const effectHandler : EffectHandler<EffectStateGet | EffectStateSet> = e => {
    switch (e.kind) {
        case 'state_get': 
            return state
        case 'state_set': 
            return state.value = e.value
    } 
}

runGeneratorSyncWithEffect(
    effectHandler,
    effectfulFunction,
    [],
    null
)

console.log(state.value) // 1